Means for reducing effects of differential cutoff

ABSTRACT

A half-tone storage tube is disclosed which, after a storage target has been electrically prepared, has information written on such storage target in a writing mode, said storage target is subjected to a small amplitude alternating voltage means while in a reading mode, thereby minimizing the effects of differential cutoff so that the image produced on the storage tube screen can easily be viewed, thereby permitting information written at higher writing rates to be viewable.

United States Patent Durecka et al.

[ Sept. 17, 1974 [54] MEANS FOR REDUCING EFFECTS OF 3,710,173 1/1973 l-lutchins et al 315/12 DIFFE CUTOFF 3,710,179 1/1973 Hayes et al. 315/12 [75] Inventors: gob: Ii ulgecka; Philig S. Crosby, Primary Examiner Maynard R Wilbur 0t 0 eaverton Assistant ExaminerJ. M. Potenza [73] Assignee: Tektronix, Inc., Beaverton, Oreg. Attorney, g FirmAdfiaH La Rue [22] Flled: May 17, 1973 ABSTRACT [21] Appl N0':361089 A half-tone storage tube is disclosed which, after a storage target has been electrically prepared, has in- [52] US. Cl. 315/12, 313/68 D forma ion written on such storage target in a writing [51] Int. Cl. H01j 29/70 m said storage get is ubjecte to a mall ampli- [58] Field of Search 315/10-12, e l rn ing ltage means while in a reading 315/13 ST; 313/68 D mode, thereby minimizing the effects of differential cutoff so that the image produced on the storage tube [56] References Cited screen can easily be viewed, thereby permitting infor- UNITED STATES PATENTS mation written at higher writing rates to be ,viewable.

3,426,238 2/1969 Gibson, Jr 315/12 6 Claims, 6 Drawing Figures 5 50 l so%v 58 k-pasiwg ro WRITE v;:w

TARGET A r2 VOLTS MEANS FOR REDUCING EFFECTS OF DIFFERENTIAL CUTOFF BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to half-tone storage tubes, and in particular, to a storage tube similar to that shown and fully described in U.S. Pat. No. 3,710,179.

In prior art, the disadvantage of the storage tube is that background areas on the display screen are of somewhat non-uniform brightness. This phenomenon is caused by deficiencies in any practical half-tone storage tube target or related structures which permit fewer flood electrons to penetrate through some areas of the mesh electrode than others. This condition is most readily seen when the voltage on the storage mesh is adjusted to near cutoff, assuming the storage surface has been prepared, i.e., erased, to present as constant a charge potential over the entire storage surface as possible.

A trace written, stored, on the above described surface very weakly, such as writing a very fast trace, produces a corresponding charge on the storage mesh. This resulting charge may be only slightly above the erased target surface. To view the trace, mesh potential is adjusted to near cutoff, but this can only be done for certain areas; all other areas will have mesh potentials above or below the cutoff value. This difference in voltage applied to the storage mesh to satisfy all areas of the target is usually referred to as differential cutoff. Thus, because of differential cutoff, the above identified trace will appear only partially written for any one voltage applied.

The differential cutoff voltage is that combination of charge potential on the storage target surface and the voltage applied to the metallic mesh at which transparency of the mesh to flood electrons is nearly zero. By varying the mesh voltage, transparency of the mesh structure to flood electrons can be varied and the resulting light output from the cathode-ray tube, herein referred to as a CRT, screen will be related. If the above relationship between mesh voltage and light output is plotted, with mesh voltage as the abcissa and light output the ordinate, for any one spot on the CRT screen the curve will be a sharp line of a particular shape as is well known in the art. Any other spot on the CRT screen will have a curve slightly different in slope, displacement, or curvature.

SUMMARY OF THE INVENTION The present invention overcomes the disadvantages of the prior art in that the effects of differential cutoff can be minimized by varying the mesh voltage with a small amplitude alternating voltage an amount corresponding to differential cutoff. If this varying voltage has a rate above the flicker rate of the eye, the entire viewing area will be uniformly written. The waveshape of the applied varying mesh voltage can be tailored to emphasize certain areas of the CRT screen surface. An extended dwell at any voltage level will preferentially favor only certain parts of the storage surface. A waveform consisting of a ramp, positive or negative, for 50 percent of the cycle will produce satisfactory results. Further, any waveform, a plurality of which are wellknown, can be used such as a squarewave of variable duty cycle, a triangular wave, a sinewave, etc.

It is therefore an object of the present invention to provide an improved image storage tube wherein the effects of differential cutoff is reduced thus permitting information written at higher writing rates to be viewed.

Another object of the present invention is to provide an improved storage tube and method of operation whereby fast writing speeds permit high brightness displays wherein unwritten background areas on the display screen are of a more uniform brightness at the higher writing speeds.

The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements.

DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a view of a CRT screen to show the effects caused by deficiencies in a typical half-tone storage tube target;

FIG. 2 is a graph of mesh voltage verses light output to produce the display shown on the CRT screen of FIG. 1;

FIG. 3 is atop view ofa storage tube according to the present invention showing irregularities contributing to differential cutoff;

FIG. 4 is a schematic view of a storage tube in accordance with the present invention;

FIG. 5 shows the voltages applied to the target in the storage tube of FIG. 4; and

FIG. 6 is an enlarged view ofa portion of the embodiment shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION As shown in the drawings, and in particular FIG. 1 and FIG. 2, the disadvantage of non-uniform background on a CRT screen are readily discernible. Areas 1, 2, and 3 as well as graphs 1, 2, and 3 are the effectcause relationship respectively, of the problem associated with prior art and of which the present invention improves, namely differential cutoff.

Differential cutoff of a transmission storage target is the difference between the maximum and minimum cutoff voltages over the entire surface of the target, and this limits the minimum voltage of any charge image which can be stored or the maximum writing speed of the storage tube. Differential cutoff voltages are typically l to l0 volts except in certain CRT transmission tubes having therein side charging operation as fully described in U.S. Pat. No. 3.7l0,l79, thus minimizing differential cutoff voltage to about 0.1 to 0.25 volts which is considerably lower than 1 to 10 volts. Thus, area 1 of FIG. I, having a mesh voltage vs. light output curve 1 of FIG. 2 will be brighter than, say, area 2 or 3 of FIG. 1 having curves 2 and 3 of FIG. 2. A trace B written on the screen shown in FIG. 1 will therefore appear lost, say, in area 1 and 3, while viewable in area 2. Better contrast between the light image display of the stored charge image and the unwritten background area occurs when the potential of charge image is therefore at the point of greatest slope on the above mentioned curves.

It was previously mentioned that deficiencies in any half-tone storage tube, or for that matter in any type of storage tube, which permit fewer flood electrons to penetrate through some areas of the internal structures then others can be shown by referring to FIG. 3 of the drawings. In this drawing the elements of the transmission tube permitting fewer electrons to strike the viewing surface of the tube are the flood guns 42, wall band electrodes 46, 47, and 48 and collector mesh electrode 40. Further, the electron tragectories E contribute to the irregularities.

As can be discerned, to view say, a very fast trace, some mesh potential at approximately cutoff must be used. This potential is therefore only exactly correct for one particular portion of the CRT; all others will be above or below this cutoff potential. Hence, even with a very small amount of differential cutoff, the viewing screen contains areas having brightness between the light image display of the stored image and the unwritten background area.

As shown in FIG. 4, one embodiment of the image storage tube of the present invention wherein the contrast between the light image display of the stored image and the unwritten background area is reduced so that information written at higher writing rates may be viewed includes a transmission type storage target 10 having a relatively thick storage dielectric layer 12 coated on the front surface ofa mesh electrode 14 connected by a lead 16 to an external voltage source, connected at point A, which applies the voltage pulses shown in FIG. to such mesh electrode. The storage dielectric 12 has a thickness greater than 5 microns and with mesh electrodes of 250 and 350 lines per inch preferably about to 30 microns to provide projecting side portions for the dielectric around the mesh opening. A related structure is described in U.S. Pat. No. 3,710,173. The phosphor screen 20 is covered by an electron permeable conductive layer 24 of aluminum which acts as an accelerating electrode for the flood electrons and is connected to an external voltage source of about 7,000 volts so that the display image emitted by the phosphor is of high brightness. It should also be noted that the brightness of the display image is further increased by the light reflecting characteristics of the conductive layer 24 in a conventional manner.

A writing gun cathode 30 connected to a high negative DC voltage of about 3,000 volts is provided at the opposite end of the storage tube envelope from the phosphor screen. The writing electrons emitted by cathode 30 are focused into a narrow beam by writing gun anodes 32 and the current density of the electron beam is determined by the negative bias voltage on a control grid 34 which may also be used as a blanking electrode to cutoff the writing beam during storage. The writing beam is transmitted between a pair of vertical deflection plates 36 and a pair of horizontal deflection plates 38 which deflect the electron beam in the conventional manner of a cathode ray oscilloscope in accordance with a vertical input signal and a ramp voltage horizontal sweep signal, respectively applied thereto to produce a charge image of the vertical waveform on the storage dielectric 12 of a target 10. The high velocity writing electrons form a positive voltage charge image on the storage dielectric 12 by secondary electron emission, such secondary electrons being collected by a collector mesh electrode 40 positioned in front of the target and connected to a positive DC voltage source of about I00 volts.

A pair of flood guns 62 having grounded cathodes 44 are provided within the storage tube to uniformly bombard the target 10 with low velocity flood electrons. A portion of these flood electrons are transmitted through the apertures of the mesh electrode 14 of storage target H0 in the written areas of such target which have been bombarded by the writing beam emitted by cathode 30. The transmitted flood electrons are caused to strike the phosphor screen 20 to produce the light image of the half-tone image stored on the target 10. A plurality of collimating electrodes 46, 48, and 50 are provided as conductive bands coated on the inner surface of the storage tube envelope which are spaced axially from each other between the flood guns 42 and the target 10. The collimating electrodes 46, 48, and 50 are respectively connected to a DC voltage source of+ 150 volts, volts, and 50 volts. These collimating electrodes provide electrostatic fields which cause the low velocity flood electrons to be uniformly distributed over the surface of the storage target 10 and to strike the storage dielectric l2 perpendicular to the storage target 10.

The transfer storage tube of the present invention is an improvement over that shown in U.S. Pat. No. 3,710,179 in that a small amplitude alternating voltage having a rate greater than the flicker rate of the eye is applied to the storage target 10 to vary the voltage on such storage target an amount corresponding to differential cutoff to make the entire written area viewable in a manner hereinafter described. It should be noted that an extended dwell of the varying voltage to the storage target at any voltage level will preferentially form only certain area of the storage surface. Hence, a waveform consisting of a ramp, positive or negative, for 50 percent of the cycle and a dwell at the lower level for the other 50 percent of the cycle produces satisfactory results.

Operation ofthe storage tube shown in FIG. 4 is best understood by referring to the voltage waveforms shown in FIG. 5. Prior to the formation of a charge image on the storage target I0 by the writing beam, a voltage 58 of about volts is applied through lead 16 to target electrode 14 which enables the storage dielectric to fade positive" to a uniform potential across the surface ofthe target, thereby erasing any previously stored charge image. Then, the pulse voltage decreases to about 5 volts so that the potential on storage dielectric 14 is brought down to the approximate level which prevents flood electrons from passing in a conventional manner.

The target is then prepared by applying preparation pulse 52 to lead 16 during the bombardment of the storage dielectric 12 by the low velocity flood electrons. The preparation pulse 52 has an amplitude of about 30 volts and is of sufficient time duration, say, 400 milliseconds to cause the storage dielectric 12 to charge to the flood gun cathode voltage before termination of the preparation pulse 52. Next, at a time 62, the storage dielectric 12 is returned to the operation level corresponding to cutoff voltage and the charge image is written on the storage dielectric 12 of the target 10 by the high velocity writing electron emitted by writing gun cathode 30 and deflected by the horizontal deflection plates 36 and the vertical deflection plates 38 in a conventional manner to form a charge image of the vertical signal waveform. Thus, the writing beam is normally cutoff and is transmitted to the storage target only during the writing time 62 when a positive unblanking voltage is applied to control grid 34 of such writing gun. Due to the high velocity of the writing electrons, the secondary emission ratio of the storage dielectric 12 is greater than unity for such writing electrons so that a charge image of positive potential is produced on the storage dielectric. As a result of this positive potential, at a time 63, flood electrons are transmitted through such target 10 in the written areas of such target to strike the phosphor screen, thereby producing a light image corresponding to the charge image. However, during the above operation, and as previously discussed, some of the flood electrons do not strike the phosphor screen. At a time 63, an alternating voltage 68 of about 1 volt peak to peak in amplitude is applied through lead 16 to storage target 10. It should also be noted that the peak to peak amplitude of the 1 volt signal compensates a particular tube having a differential cutoff of one volt and is therefore not limited to 1 volt.

Operation of the storage tube will be herein described and can best be understood by referring to FIG. 6. During write time, a plurality of flood electrons 54 are deflected through target 10 in the written areas of such target to strike the phosphor screen, thereby producing a light image corresponding to the charge image in a conventional manner. However, due to the irregularities ofthe storage tube, some flood electrons are directed away from target 10, hence never reach the phosphor screen. As a result, the trace written on the phosphor screen can only partially correspond to the charge image on storage dielectric 12. Now however, voltage 68, applied to storage dielectric l2, raises or lowers the overall charge image on storage dielectric 12 in a linear manner so that at some point on such alternating waveform flood electron previously inhibited from striking the phosphor screen are now deflected to strike the phosphor screen. As a result, the effects of differential cutoff are minimized and the image or trace on the storage tube screen can easily be viewed, thereby permitting information written at higher writing rates to be viewed.

While there has been shown and described the preferred embodiments of the present invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing therefrom in its broader aspects. For example, a second target including a storage dielectric coated on the front surface of a mesh electrode, such dielectric being of less thickness than the first target dielectric 12, can be placed between the collector 40. Further, pulse 68 may be any ofa plurality of waveforms well known by those skilled in the art. Therefore, the appended claims are intended to cover all such changes and modifications as fall within the true spirit and scope of this invention.

The invention is claimed in accordance with the following:

l. A charge image storage tube in which the improvement comprises:

a storage target including a mesh electrode having a plurality of mesh openings therethrough and a storage dielectric layer;

means for uniformly bombarding the storage dielectric with low velocity electrons emitted by a first cathode;

writing means for bombarding the storage dielectric at written target areas with a writing beam of high velocity electrons emitted from a second cathode more negative than said first cathode to form a positive charge image thereon which enables the low velocity electrons to be transmitted through said written areas;

means for preparing said storage target for enabling said storage target to produce said positive charge image prior to formation of said charge image thereon;

means for changing said charge image on said storage dielectric whereby said low velocity electrons are uniformly transmitted through said written'areas; and

means for producing a light image corresponding to said charge image in accordance with said low velocity electrons transmitted through said written areas.

2. A storage tube in accordance with claim 1 wherein said means for producing a light image is a phosphor screen positioned on the opposite side of the storage target from said first cathode which is a flood gun cathode.

3. The storage tube according to claim 2 wherein said means for preparing said storage target defines means for applying a preparation voltage pulse to said mesh electrode prior to formation of said charge image thereon but during bombardment of the storage dielectric by the low velocity electrons.

4. The storage tube according to claim 1 wherein said means for changing the charge image on the storage dielectric defines means for applying a small amplitude alternating voltage pulse to said mesh electrode following the formation of the charge image on the storage dielectric by the low velocity electrons said alternating voltage having an amplitude corresponding to differential cutoff.

5. A method of operating a charge image storage tube having a storage target including a mesh electrode in which this improvement comprises:

bombarding the storage dielectric substantially uniformly with low velocity electrons emitted by a first cathode;

applying a preparation voltage pulse to the mesh electrode during the bombardment of the storage dielectric by the low velocity electrons to prepare the target for formation of a charge image thereon;

bombarding the prepared storage dielectric with a writing beam of high velocity electrons emitted by a second cathode more negative than said first cathode to form a charge image on written areas;

6. The method in accordance with the method of claim 5 in which the storage tube includes a phosphor screen and the low velocity electrons are transmitted through the written areas of the storage target to the phosphor screen to produce the corresponding light image on the phosphor screen. 

1. A charge image storage tube in which the improvement comprises: a storage target including a mesh electrode having a plurality of mesh openings therethrough and a storage dielectric layer; means for uniformly bombarding the storage dielectric with low velocity electrons emitted by a first cathode; writing means for bombarding the storage dielectric at written target areas with a writing beam of high velocity electrons emitted from a second cathode more negative than said first cathode to form a positive charge image thereon which enables the low velocity electrons to be transmitted through said written areas; means for preparing said storage target for enabling said storage target to produce said positive charge image prior to formation of said charge image thereon; means for changing said charge image on said storage dielectric whereby said low velocity electrons are uniformly transmitted through said written areas; and means for producing a light image corresponding to said charge image in accordance with said low velocity electrons transmitted through said written areas.
 2. A storage tube in accordance with claim 1 wherein said means for producing a light image is a phosphor screen positioned on the opposite side of the storage target from said first cathode which is a flood gun cathode.
 3. The storage tube according to claim 2 wherein said means for preparing said storage target defines means for applying a preparation voltage pulse to said mesh electrode prior to formation of said charge image thereon but during bombardment of the storage dielectric by the low velocity electrons.
 4. The storage tube according to claim 1 wherein said means for changing the charge image on the storage dielectric defines means for applying a small amplitude alternating voltage pulse to said mesh electrode following the formation of the charge image on the storage dielectric by the low velocity electrons said alternating voltage having an amplitude corresponding to differential cutoff.
 5. A method of operating a charge image storage tube having a storage target including a mesh electrode in which this improvement comprises: bombarding the storage dielectric substantially uniformly with low velocity electrons emitteD by a first cathode; applying a preparation voltage pulse to the mesh electrode during the bombardment of the storage dielectric by the low velocity electrons to prepare the target for formation of a charge image thereon; bombarding the prepared storage dielectric with a writing beam of high velocity electrons emitted by a second cathode more negative than said first cathode to form a charge image on written areas; applying a small amplitude alternating voltage pulse to said storage dielectric after the formation of said charge image whereby the charge image changes in response to said voltage so that an offset in differential cutoff is obtained; and producing a light image corresponding to said charge image in accordance with said alternating voltage.
 6. The method in accordance with the method of claim 5 in which the storage tube includes a phosphor screen and the low velocity electrons are transmitted through the written areas of the storage target to the phosphor screen to produce the corresponding light image on the phosphor screen. 